In 2014 the Retroviral Immunology Section continued investigation into host mechanisms of genetic resistance and susceptibility to retroviral infection. Following is a brief summary of five publications in peer-reviewed journals describing our discoveries in this area in the 2014 fiscal year. These findings have implications for the design of therapeutics and vaccines to treat and prevent infections with viruses such as HIV. 1. The role of immune cells changes during the course of infection, especially CD8+ T cells that are necessary to clear acute infections, but which can become immunopathogenic if their activity continues too long. Thus it is important to understand the kinetics of the CD8+ T cell response and the different cell surface markers that distinguish various forms of these effector cells. We found that CD8+ T cells produced multiple antiviral cytokines, cytotoxic granules and displayed markers of recent cytolytic activity during acute infection, despite the presence of cell surface markers considered to be inhibitory. Furthermore, we demonstrated high levels of virus-specific cytotoxicity in vivo. We also showed that mice genetically deficient in CD8+ T cells had much higher virus loads than wild type mice and that mice could not survive acute FV infection without CD8+ T cells. The take-home message from these studies is that expression of inhibitory receptors does not independently indicate loss of function. Rather, the functionality of CD8+ T cells is dependent on the balance of signals from inhibitory receptors along with positive signaling through the TCR and costimulatory molecules. In the inflammatory environment of acute infections the balance is strongly in favor of antiviral function. In FV infection this balance only shifts when regulatory T cells expand and the infection becomes chronic (1). 2. In order to further our studies of FV-specific CD4+ T cell responses during FV infections we recently analyzed the antigenic specificity and breadth of these responses in resistant mice. Nine new CD4+ T cell epitopes were identified, 3 encoded by gag, 1 by pol, and 5 by env. These results greatly expand the number of known H-2b-restricted FV epitopes from 1 to 10. The high resistance of H-2b mice is likely related to this very broad CD4+ T cell response against multiple peptides during FV infection. We are now examining the repertoire of peptides recognized by susceptible mouse strains and mice with genetic inactivations such as mouse APOBEC3 KO mice. Interestingly, we found that mouse APOBEC3 expression significantly influences the magnitude and kinetics of both CD4+ and CD8+ T cell responses with no qualitative changes in epitope specificity (2). 3. APOBEC3/Rfv3 and its function have been of great interest because it controls neutralizing antibody responses to retroviral infection. In an exciting new manuscript we provide evidence that the affinity maturation of Friend virus-specific B cells is in part due to the deoxycytidine deaminase activity of APOBEC3, which promotes somatic hypermutation of immunoglobulin genes in germinal centers. This is the first description of an enzyme other than activation-induced deaminase (AID) shown to be involved in somatic hypermutation of immunoglobulin genes (4). 4. The mechanisms whereby different vaccines may expand distinct Ag-specific T cell clonotypes or induce disparate degrees of protection are incompletely understood. We found that several delivery modes of a model retroviral Ag, including natural infection, preferentially expanded rare high-avidity CD4(+) T cell clonotypes, known to mediate protection. In contrast, the same Ag vectored by human adenovirus serotype 5 induced clonotypic expansion irrespective of avidity, eliciting a predominantly low-avidity response. Mechanistically, the potency of Ag presentation determined the speed and, consequently, completion of the CD4(+) T cell response. These results highlighted the importance of Ag presentation patterns in determining the clonotypic composition of vaccine-induced T cell responses and ultimately the efficacy of vaccination. The results are particularly relevant to HIV vaccines vectored by Ad5 constructs (3) 5. Tetherin/BST-2 is a host restriction factor that can directly inhibit retroviral particle release by tethering nascent virions to the plasma membrane in vitro. However, the immunological impact of Tetherin during retrovirus infection remains unknown. We now show that Tetherin influences antiretroviral cell-mediated immune responses. In contrast to the direct antiviral effects of Tetherin, which are dependent on cell surface expression, the immunomodulatory effects are linked to the endocytosis of the molecule. Mice encoding endocytosis-competent C57BL/6 Tetherin exhibited lower viremia and pathology at 7 d postinfection with Friend retrovirus (FV) compared with mice encoding endocytosis-defective NZW/LacJ Tetherin. Notably, antiretroviral protection correlated with stronger NK cell responses. In addition, Friend retrovirus infection levels were significantly lower in wild-type C57BL/6 mice than in Tetherin knockout mice at 2 wk postinfection, and antiretroviral protection correlated with stronger NK cell and virus-specific CD8+ T cell responses. The results demonstrate that Tetherin acts as a modulator of the cell-mediated immune response against retrovirus infection in vivo (5).